Here is a list of the best Google Chrome Search Engine Optimization (SEO) extensions (plugins).

Search Engine Optimization (SEO) Extensions

Chrome SEO

The Google Chrome SEO Extension. Make your SEO tasks easier with Chrome SEO Tools, includes ‘all’ the stats and data you need.
Install Extension

SEO Status Pagerank/Alexa Toolbar

SEO Status is lightweight. It will check the Google Pagerank (PR) & Alexa rank for every page. Using this will increase your Alexa.
Install Extension | Developer website

PageRank

Fastest and Very light weight. It shows Google page rank on toolbar.
Install Extension | Developer website

PageRank Status

Shows Google PageRank and Alexa Rank of the current web page, quick access to Whois, Alexa, backlinks and indexed pages.
Install Extension | Developer website

SEO Site Tools

Pulls tons of On-Page and Off-Page metrics, Social Media info and enhances SERPs with metrics as well as GA, WMT and much more.
Install Extension | Developer website

META SEO inspector

Useful to inspect the meta data found inside web pages, usually not visible while browsing.
Install Extension | Developer website

SEO SERP

A simple tool to quickly check the position of a list of sites given a keyword.
Install Extension | Developer website

SEO

This Chrome SEO plugin implements the SEO Playground Tool (Bookmarklet) by David Naylor. Making it easier to use in Chrome.
Install Extension | Developer website

iPageRank | SEO Tool for Chrome

This plugin gets PageRank & calculates how much PR is transfered to external links, analyzes keywords, Alexa, TIC. More inside.
Install Extension | Developer website

SeoQuake

SeoQuake is a SEO extension (toolbar) for Google Chrome, it shows PageRank, Alexa and tons of SEO parameters, also works in SERP.
Install Extension | Developer website

SitezMeter

SitezMeter displays traffic charts from Google Trends, Alexa and Compete side by side, as well as ranks.
Install Extension | Developer website

Chrome Flags

Porting of FlagFox to Chrome. Displays a country flag indicating the location of the websites you’re visiting.
Install Extension

I update the list whenever I find a good Google Chrome Search Engine Optimization SEO extensions. If you know of other good SEO extensions for Google Chrome, please let me know, so I add them to the list.